It is known that hearing aids may operate in acoustic coupling mode or telecoil coupling mode. Hearing aids operating in acoustic coupling mode can receive and amplify sounds surrounding the user, both desired sounds, such as a telephone's audio signal, as well as unwanted ambient noise. Hearing aids operating in telecoil coupling mode may avoid unwanted ambient noise by turning off a hearing aid microphone and receiving only magnetic fields generated by telecoil-compatible telephones.
Although analog cellular radiotelephones do not generally cause interference problems for hearing aid users, digital cellular radiotelephones may cause interference with hearing aids and cochlear implants because of the electromagnetic energy emitted by some of the radiotelephone components. This interference can be significant enough to prevent individuals with hearing aids or cochlear implants from using digital cellular radiotelephones. In addition, some cellular radiophones may not provide the capability to inductively couple with hearing aids containing telecoils.
In particular, some digital cellular radiotelephone systems use Time Division Multiple Access (TDMA) techniques for communications, whereby a single radio frequency channel is shared between several calls by allocating different transmit and receive timeslots to different calls. Additionally, the transmit and receive timeslots in these TDMA systems are staggered so that the phone may not need to transmit and receive simultaneously so that the transmitter is inactive for a certain period during each frame.
The loss of transmission time (i.e., the inactive time of the transmitter) can be compensated for by buffering the digitized speech signal and subsequently transmitting the buffered signal at a higher rate than that at which it was buffered during its allocated slot. The transmitter power may be increased to support the higher rate. When such a transmitter is operated near sound reproducing equipment, such as a hearing aid, the switched operation of the transmitter may affect operation of the hearing aid. For example, the transmitter may induce an audible buzz in the audio generated by the hearing aid.